Red gemstone (4)
I believe the answer is:
ruby
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">red gemstone</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for ruby that I've seen before include "Deep 6 across" , "Deep red variety of corundum" , "Stony girl" , "Completely" , "Gemstone - shade of red" .)